The primary objective of this phase III trial is to evaluate the safety and efficacy of aminoguanidine in preserving renal function in patients with insulin dependent diabetes mellitus and overt diabetic nephropathy. Secondary objective include, 1) to evaluate the effect of aminoguanidine on protein crosslinking in diabetic patients as measured by circulating blood and urinary concentrations of advanced glycated endproducts (AGEs), 2)to evaluate the relationship between plasma aminoguanidine concentrations and primary and secondary efficacy endpoints as well as adverse events.